The invention pertains to a liquid, silica-based binder suitable for use in the manufacture of refractory molds, the silica base comprising at least two types of silica sources, one of them being colloidal silica, and the other being a prehydrolyzed organosilicate.
Silica-based binders are frequently used for the bonding of refractories in the metals investment casting process. The particular binder of the aforementioned type is commonly referred to as a hybrid binder on account of the combination of the two silica sources. One such binder has been disclosed in U.S. Pat. No. 3,576,652 to Teicher et. al.
In general, hybrid binders are utilized in view of their combining the favorable characteristics of the two silica sources while suppressing their drawbacks. The prehydrolyzed organic silicate, more particularly, prehydrolyzed organic ethyl silicate, is chosen for its high refractoriness and rapid drying. Advantages of colloidal silica sols include high bond strength and good slurry stability.
The hybrid binders used at present in foundries are fastcuring by virtue of the presence of prehydrolyzed organosilicate. To dissolve the organosilicate, the conventional hybrid binders essentially comprise a volatile organic solvent, such as ethanol.
It is commonly recognized that the presence of water in organosilicate-containing binders should be prevented. Water residues are known to impair the binder's stability, i.e., to cause gelation. Unstable binders would require immediate processing. Since in foundries it is generally preferred not to prepare the binder in situ, also because most foundries do not have the appropriate equipment, the state of the art binders delivered to foundries are organic solvent based In line herewith, the Teicher disclosure referred to above describes systems which are virtually water-free.
Though organic solvents are necessary for obtaining a stable and satisfactory product, releasing large amounts of volatiles into foundries is no longer acceptable for health and environmental reasons This forms a serious drawback to the use of state of the art fast-curing hybrid binders.